Electrothermally concentrating an aqueous electrolyte

ABSTRACT

There is described an electrothermal cell provided with a region of reduced cross-sectional area, measured at right angles to the current path between the electrodes, compared with the crosssectional area of the cell in the region of the electrodes. Such a cell displays good characteristics during electrolysis. In particularly there is little vapor formation in the region of the electrodes. A process of heating or concentration using the cell is also described.

United States Patent White et al.

[151 3,664,929 51 May 23, 1972 [541 ELECTROTHERMALLY CONCENTRATING AN AQUEOUS ELECTROLYTE [72] Inventors: Arnold George White, Trail, British Columbia; Thomas Edward Smith, Rossland, British Columbia; Lyall Campbell Work, Trinity Bay, Newfoundland; Willard Wallace Miller, Rossland, British Columbia, all of Canada [73] Assignee: Cominco Ltd., Montreal, Quebec, Canada 22 Filed: Aug. 8, 1969 21 Appl. No.: 848,486

[30] Foreign Application Priority Data Aug. 9, 1968 Canada ..O27,209

[52] US. Cl. ..203/l0, 159/D1G. 1, 159/47, 202/234, 203/100, 219/275, 219/284 [51] Int. Cl. ..Cl0b 3/00 [56] I References Cited UNITED STATES PATENTS 1,685,266 9/1928 Baum ..219/284 1,815,978 7/1931 Hitner.... ..13/6 2,272,345 2/1942 Kobe ..23/ l 21 3,328,153 6/1967 Augsburger... ..13/6 X 3,349,160 10/1967 Rapson 219/284 X 3,388,205 6/1968 l-laavik et al ..l3/6 X Primary Examiner-Norman Yudkofi' Assistant Examiner-David Edwards Attorney-Smart & Biggar ABSTRACT There is described an electrothermal cell provided with a region of reduced cross-sectional area, measured at right angles to the current path between the electrodes, compared with the cross-sectional area of the cell in the region of the electrodes. Such a cell displays good characteristics during electrolysis. ln particularly there is little vapor formation in theregion of the electrodes. A process of heating or concentration using the cell is also described.

6 Claim, 6 Drawing figures Patented May 23, 1972 3,664,929

2 Sheets-Sheet l v ELECTROTHERMALLY CONCENTRATING AN AQUEOUS ELECTROLYTE BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to an improved process and apparatus for electrothermal heating or concentrating of electrolytes.

2. Description of the Prior Art It has long been known to concentrate solutions of electrolytes by the direct application of an electrical current with the solution itself sewing as resistor. Both direct and altemating currents have been used successfully. Various devices have been described in the literature, such as that shown in ,Baum, U. S. Pat. No. 1,688,680, issued Oct. 23rd, I928. Other examples of prior art devices include those described in Kobe, US. Pat. No. 2,272,345, issued Feb. 10, 1942. These prior art devices are all concerned with providing uniform heating of the electrolytes.

In our co-pending White et al. US. application Ser. No. 829,732, filed June 2, 1969, we have disclosed a process for the concentration of phosphoric acid, wherein an alternating current is applied to electrodes placed in the acid and wherein the formation of scale on the electrodes is minimized by operating this process within a narrow range of electrode current densities. Below the lower limit of this range the amount of scaling which occurred made operating the process in a continuous manner in practical, while above the higher limit of this range other undesirable effects became increasingly noticeable. These other undesirable effects are manifested in the form of ablation and erosion of the electrodes which may be caused bythe formation of vapor at the electrode surface and the occurrence of arcing.

Arcing is the phenomenon whereby an electrical discharge takes place through a conducting vapor path between two points maintained at different potential. Arcing can thus take place in the electrothcrmal heating and concentrating of electrolytes between the electrode surface and the electrolyte being heated, due to separation of the electrolyte from the electrode surface. Arcing can also occur when vapor is entrapped between the electrode and the walls of the apparatus. The effects of arcing and vapor formation in the electrolyte, especially'in the regions of the electrodes, result in material damage to the electrodes and the apparatus, in a decreased efficiency of the process and in possible formation of undesirable by-products.

SUMMARY OF THE INVENTION We have now discovered that if a cell containing at least two electrodes is designed in such a manner that a region is provided between the electrodes in which the cross-sectional area of the electrolyte, normal to the current path between the electrodes, is less than the corresponding cross-sectional area of the electrolyte in the region of the electrodes, the formation of vapor in the region of the electrodes is almost entirely prevented and arcing and other undesirable electrical side-effects are eliminated.

BRIEF DESCRIPTION OF DRAWINGS Embodiments of the invention are illustrated in the attached drawings, in which FIG. 1 is a plan in section of one embodiment of the cell of the invention;

DESCRIPTION OF THE PREFERRED EMBODIMENTS In a cell in which the electrolyte has a uniform cross-sectional area, the resistance of the electrolyte is unifonn over the distance between the electrodes and the voltage drop is gradual. Consequently, boiling occurs through the entire length of the cell. However, if a region is provided with a reduced liquid cross-sectional area, the resistance of the liquid in this region increases and the power dissipation is no longer uniformly distributed over the distance between the electrodes but shows an increase in the region which has a'reduced liquid cross-sectional area. This results in an increased power dissipation and boiling in this region. It is thus possible to direct the power dissipation in the cell to a desired location. By providing the region with a reduced-liquid cross-sectional area in the central portion of the cell it is possible to direct most of the power dissipation to this location, with the results that boiling in the region of the electrodes is almost entirely eliminated, contact between the electrolyte and the electrodes and the walls of the cell is much improved and no arching takes place at the electrodes.

Another important advantage of providing the reduced liquid cross-sectional area in the cell is the stabilization of the operation of the process. When operating the process in an ordinary cell the power input will fluctuate because of variations caused by the uneven boiling taking place throughout the cell, while when the region of reduced liquid cross-sectional area is provided in the cell the conditions are steady, the power input does not fluctuate and the operation is stabilized.

The region of reduced liquid cross-sectional area can be provided in a number of ways. Thus, inserts can be placed in the cell to create a constriction, or the cell walls can converge towards the center of the cell, or a pool can be created at the electrodes with a greater depth than the liquid in the central portion of the cell. The pool of liquid can be conveniently created by providing tilting cell sections containing the electrodes or providing inclined sections in the cell bottom.

Particularly with the use of the tilted cell sections or the inclined bottom sections, boiling of the electrolyte at the electrodes is almost entirely eliminated, so that substantially all of the boiling takes place in the central portion of the cell, i.e., in the region of reduced liquid cross-sectional area. The inclined bottom is not necessary over the entire length of the cell bottom, and can be provided adjacent the electrodes with a horizontal section between the inclined sections.

According to a particularly preferred feature of the invention the cell consists of at least two sections or legs each having a closed end and an open end, and containing an electrode at each closed end. Each leg or section has a bottom sloped downwardly towards the closed end and horizontal towards the open end. The legs or sections are joined at the open end to form the center of the cell in which a product discharge port is provided.

It was found during the operating of the cell of the invention that vigorous boiling occurred within the liquid in the cell in the region with the reduced liquid cross-sectional area only, i.e., in the central portion of the cell and this resulted in a minimum retention time of the most concentrated electrolyte in the cell. The vigorous boiling in this region of the cell also created considerable internal circulation of the cell-contents which minimized any possible accumulation of sludge.

The length and the width of the cell legs are determined by considering the retention time required for a particular cell throughout, the conductivity of the electrolyte, the available voltage supply, and the required power input.

The concentrated electrolyte produced in the cell can be discharged by either an overflow or an underflow discharge.

The overflow discharge is less desirable, as accumulation of sludge in the cell may occur. In addition, an overflow discharge could result in by-passing of the lighter feed liquid to the product discharge, thus diluting the product.

An underflow discharge reduces any possibility of bypassing in the cell, and also reduces the possibility of accumulation of sludge. The preferred design for an underflow discharge includes the incorporation of a conical outlet in the center of the cell. This also overcomes the difficulty caused by the high degree of agitation and bubble formation which exists in the region with reduced liquid cross-sectional area and which hinders a smooth discharge of the product from the cell. The discharge may, for example, be a simple flexible U-tube attached to the conical section, which can be used for adjusting the liquid level in the cell, and which is provided with an antisiphon to prevent siphoning of the liquid from the cell.

Materials used to construct the cell must be carefully selected because of the nature of the process and the chemical compounds handled. Materials suitable for the construction of the cell include those that are inter alia corrosion-resistant, have a low electrical conductivity, are non-porous and can withstand prolonged operation at elevated temperatures. The choice of material is dependent on the type of electrolyte being concentrated and its physical properties. Among the suitable materials for the interior of the cell are fiber reinforced polyester, polytetrafluoroethylene and polypropylene.

It is possible to use electrical power of varying voltages, currents and number of power phases with the apparatus of the invention. Thus, for example, single-phase or three-phase power of 220 or 550 volts may conveniently be used. In a balanced three-phase system, identical loads are connected in each phase and these may be connected either in Y or A- formation. In a Y connection the line current is equal to the phase current and the line voltage equals the phase voltage multiplied by 3. In a A- connection the line voltage equals the phase voltage and the line current is equal to 3 times the phase current. Thus, it is possible to use two electrodes and single-phase power or three electrodes and three-phase power.

When single-phase power is used as the source of electrical energy supplied to the electrodes, the cell can conveniently have an oblong shape consisting of two sections or legs each having a closed end and an open end, and each leg having a bottom inclined downwardly towards the closed end and horizontal towards the open end. The two legs are joined at the open ends to form the common area of the cell. The product discharge port is provided at the center of the cell.

When the multi-phase system is used, the configuration of the cell is arranged in such a manner that one leg is provided for each power phase of the multi-phase input. Thus, for example, when three-phase power is used, three longitudinal sections or legs are provided, each having a closed end and an open end and each having a bottom inclined downwardly towards the closed end and horizontal towards the open end. These three legs are then joined at the open end to form a common area at the center of the cell with the closed end remote therefrom. The product discharge port is provided in the center of the cell. The legs of the multi-phase system are at the maximum voltage difference obtainable, without undesirable electrical effects occurring at the electrode situated in each leg. It will, of course, be appreciated that other multiphase input systems are possible, and that the cell can be formed of a number of legs equivalent to the number of phases of the input. The cell center in a multi-phase system is nearly neutral to ground, and it can be suitably grounded to prevent power losses through the the discharge outlet and other connected, external equipment.

It was found that the power input into the cell can be regulated by the depth of the electrolyte in the cell. For example, when single-phase power was supplied to two electrodes placed in a cell containing an electrolyte at successive decreasing depths, the current drawn and the power input dropped accordingly.

The electrolyte to be heated and/or concentrated can be fed either into the center of the cell or in the regions of the electrodes, and can be supplied, for example, from a feed tank. The temperature of the electrolyte-feed should be at least several degrees below that of the electrolyte in the cell, resulting in a temperature gradient in the cell contents.

In the case of the cell feed being supplied in the regions of the electrodes, the temperature in the regions of the electrodes is lower than that in other parts of the cell. This results in a number of important advantages.

Thus, due to the electrical resistance of the electrode material some resistance heating will occur, which will result in an increased electrode surface temperature and this can be the cause of boiling at the electrode. By feeding the relatively cool electrolyte in the regions of the electrodes, the surface temperature of the electrodes is sufiiciently lowered to suppress any boiling. Any arcing that might be initiated by electrode surface boiling is thereby prevented. Also, any possible by-passing of the electrolyte feed to the product discharge is eliminated.

Stainless steel electrodes can be advantageously used in the concentration of slightly corrosive or non-corrosive materials and especially in the concentration of heat sensitive materials.

Graphite electrodes have been found to be particularly suitable for use in the cell of this invention in that they are conductive, chemically inert, readily available, and easily fabricated. Graphite electrodes are especially suitable in highly corrosive electrolytes, e.g., wet process phosphoric acid.

The process is operated at certain current densities on the immersed portion of the electrodes, their values for obtaining optimum conditions being determined mainly by the properties of the electrolyte being processed.

It was also found that the electrodes should be deeply immersed in the electrolyte. With a shallow immersion of the electrodes, evolution of water vapor at the electrode-liquid interface easily takes place, resulting in a parting of the liquid film from the electrode surface, which can lead to arcing with consequent damage to the electrode. Deep immersion of the electrodes tends to prevent this condition.

It is also important to maximize the clearance between the electrode and the cell wall to protect the cell wall from possible arcing.

The process and apparatus of the invention can be used in a number of applications. Its primary use is for heating and concentrating electrolytes.

It can also be used to heat and concentrate an electrolyte causing evolution of a gaseous phase from the electrolyte, which gaseous phase can be separated from the liquids and recovered as one of the products.

It is also possible to heat and concentrate an electrolyte with the electrolyte being crystallized from the concentrated electrolyte after the heating and concentration process. Furthermore, it is possible to heat and concentrate mixtures of electrolytes with one or more of the electrolytes being crystallized from the concentrated mixture of electrolytes.

The process can also be used for heating or for both heating and concentrating of non-electrolytes which have been made conductive by the addition of a small amount of an electrolyte.

The process and apparatus of this invention are applicable to a wide variety of electrolytes. Examples of such electrolytes include solutions containing salts such as (NR S0 and Zn- SO mixed-salt solutions such as seawater and brines, acids such as sulphuric acid and phosphoric acid, and hydroxides such as sodium hydroxide.

Examples of electrically non-conductive liquids which can be treated after adding an electrolyte to make the liquid conductive are methanol-water mixtures containing a salt to evaporate methanol and urea-water mixtures containing a salt to concentrate urea solutions.

Referring to the drawings, FIG. 1 shows a cell having three electrode compartments or legs, A, B and C emanating from the center of the cell at identical angles of Because legs A, B and C are identical, only a typical leg A will be described hereinafter.

As shown in FIGS. 2 and 3, each leg is made up of two parallel and vertical side walls 1 spaced from each other, one end wall consisting of a vertical section 2 and a slanted section 3, which sections are attached onto and at one end of the two side walls 1, said one end being the end farthest removed from the center of the cell, and a bottom plate consisting of a horizontal section 4 at the central portion of the cell and an inclined section 5, both sections attached onto the side walls 1, while the canted section 5 is also joined onto section 3 of the end wall. Electrode 6 is rigidly suspended in a vertical position parallel to vertical section 2 of the end wall and both side walls 1. A bolt 7 or other conventional means is used to suspend the electrode, which bolt or other means protrudes through vertical section 2. The power -supply lead-wire 8 is attached to the protruding end of bolt 7. At the center of the cell a conical discharge 9 is fitted into and attached to the horizontal section 4 of the bottom plate. An inlet pipe 14 is provided for feeding electrolyte to each leg. The other legs B and C are joined in like fashion to the conical discharge and the side walls of the legs are joined to form the configurational cell as shown in FIG. 1.

The individual legs thus have each a closed end which contains the electrode, which is thereby spaced as far as possible from the center of the cell, and an open end in the central portion of the cell. When the individual legs are joined to form the configurational cell, said open ends form a central portion or area 10 which surrounds the center of the cell. Thus the elongated portions, or legs, have their closed ends remote from the common'area and merge with their open ends into said common area.

As shown in FIG. 4, the product discharge from the cell consists of a length of flexible pipe 11 which is crimped onto the bottom of discharge cone 9 and an overflow 12 attached to pipe 11 and which contains an antisiphon device 13.

Means, not shown, are normally provided for the treatment of vapors and/or fumes which are evolved from the electrolytes being processed according to the invention, whereby valuable by-products can be recovered and air-pollution be minimized.

The process can be operated at super-atmospheric, atmospheric or subatmospheric pressure; the last will facilitate the treatment of vapors and/or fumes, as mentioned above.

The lower limit of the angle of the inclined section of the bottom (see FIG. 2) is fixed by the fact that at an angle of 0' no reduced liquid cross-sectional area is obtained in the cell and the power dissipation is evenly distributed throughout the liquid between the electrodes in the cell. The upper limit of the angle is determined by the fact that a sharp voltage drop exists at the point of incline and the steeper the angle the greater the voltage drop and this voltage drop must be less than the minimum voltage necessary todraw an arc between the liquid and the electrode or cell wall.

FIGS. 5 and 6 show an alternative embodiment of the invention with two electrodes and a flat bottom.

This cell has two end walls connected by side walls having central parallel portions 22 and portions 21 converging between the end walls 20 and the parallel portions 22. The cell has a flat bottom 23.

Electrodes 25 are vertically mounted adjacent the end walls 20 by means of mounting posts 26. The electrodes can be connected to a suitable power supply by connecting means not shown.

This arrangement provides a central portion 29 of reduced cross-sectional area relative to end regions 30 in the vicinity of electrodes 25.

An outlet 24 is provided in bottom 23 in the central portion 29 while inlet pipes 27 are positioned near the electrodes 25.

The invention is illustrated further by the following nonlimitative examples.

It will be understood that the cited figures for the depth of immersion of the electrode in the electrolyte were obtained by measurement under static conditions and that current densities were calculated on the basis of the so defined depth of electrode immersion. The duration of the tests illustrated by the following examples was from 4-l2 hours or longer. All percentages are by weight.

EXAMPLE 1 An oblong cell, 40 inches long by 2 inches wide and 8 inches high with a flat bottom made from polypropylene was used. The cell was equipped with an overflow discharge. In the cell were placed at opposite ends two graphite electrodes 1 onehalf inch wide, one-half inch thick and 8 inches long. The electrodes were'immersed to a depth of 1 inch in phosphoric acid containing 50% P 0, which filled the cell to a depth of 2 inches.

Single-phase power at an average 558 volts at 29 amperes was applied to the electrodes for a measured power input of 16.2 KVA at a measured power factor of unity. For the given depth of immersion of the electrodes in the acid the current density on the immersed portion of the electrodes was 6.1 A/sq. in. Wet process acid containing 30% P 0, was fed to the cell at a rate of 430 ml/min and concentrated acid containing approximately 50% P 0 was withdrawn at a rate of 2 10 ml/min. The average temperature of the acid in the cell was C. Uneven boiling took place in the cell and arcing occurred at the electrodes.

EXAMPLE 2 In this example the'polypropyleneoblong cell used in Example 1 was altered to incorporate a canted bottom and a variable underflow discharge port to control the level of the liquid in the cell. The liquid level in the cell was 1 V4 inches in the region of reduced liquid cross-sectional area and 3 V4 inches at the deepest point, i.e., at the opposite ends of the cell. In the cell at the opposite ends were placed two graphite electrodes 1% inches wide, one-half inch thick and 8 inches long in such a way that at least one-quarter inch clearance existed between the walls and bottom of thecell and the electrode surface.

Under the same operating conditions as in Example 1, boiling of the phosphoric acid in the regions of the electrodes was almost entirely eliminated and took place substantially in the region of reduced liquid cross-sectional area. Arcing at the electrodes was eliminated.

EXAMPLE 3 In the cell of Example 2 and under the same operating conditions, the depth of immersion of the electrodes in the acid in the cell was varied. The currents measured (see Table l showed that the power input to the cell is virtually independent of electrode immersion.

In the cell of Example 2 and under the same operating conditions and with a constant depth of immersion of the electrodes, the depth of the liquid, measured in the center of the cell, was varied (See Table 2). It was thereby shown that the power input into the cell can be regulated by the depth of the liquid in the cell.

TABLE 2 Power input, voltage Liquid depth Current measured constant at 560V 52" 25A 14.0 KVA 1%" 36A 20.2 KVA 2%" 51A 28.6 KVA EXAMPLE 5 The cell was a Y shaped polypropylene cell as'shown in the accompanying drawings and consisted of three, equiangularly positioned legs, each leg being 28 inches long and 2 inches wide and decreasing in depth from 18% inches at the electrode to 16 inches at a point 6 inches from the center of the cell and of constant depth of 16 inches from this point to the center of the cell. A conical bottom discharge was positioned in the center of the cell and was provided with a flexible line for adjustment of the liquid level in the cell. The three graphite electrodes placed in the cell as shown in the drawings were 0.35 inch diameter and 20 inches long and were immersed to a depth of 3 inches into the liquid in the cell. The depth of the liquid in the central portion of the cell was 1 /2 inches.

Fresh dilute phosphoric acid containing 30% P specific gravity 1.36, was charged at the electrodes from three separate feed lines at equal rates for a total flow of 1,160 ml/min. The power applied to the electrode was nominal 550 volt three-phase power to give an actual voltage of 575 volts at a current 34 amperes, giving a current density of 10.0 A/sq.in. on each electrode for a power input of 34 KVA. From the cell was discharged a flow of 555 ml/min of concentrated phosphoric acid containing 50% P 0 specific gravity 1.70. The average temperature of the acid measured at the electrodes was 134 C., while the average temperatures of the acid at the center of the cell and also of the cell discharge stream were measured at 137 C.

The voltage to ground of the feed tank was approximately 7 volts and that of the product discharge stream was measured to be 25 volts. The process operated continuously without arcing taking place and with a minimum of scale formation on the electrodes.

EXAMPLE 6 An oblong cell, 40 inches long, 2 inches wide and 9 inches high was used with a bottom canted in a way that the center of the cell was raised 2 inches above the ends of the cell. The cell was equipped with an overflow discharge positioned at the center of the cell, at a point 2 inches from the bottom. The depth of electrolyte in the cell was 2 at the center of the cell and 4 inches at the opposite ends of the cell. A graphite electrode with a cross section of one-half inch square was placed in each end and immersed to a predetermined depth in the electrolyte.

The electrolyte was an aqueous solution of (N1-I.,) SO which was concentrated up to the point of incipient crystallization. The electrodes were immersed to a depth of 2 inches.

Single-phase power at 200 volts and 37 amperes was applied to the electrodes. The current density on the immersed portion of the electrodes was 8.7 A/sq.in.

An aqueous solution containing 38.6% (NI-1,),SO, was charged continuously at the electrodes from two separate feed lines at equal rates, for a total flow of 40,092 gram per hour of feed solution. From the cell was discharged 31,740 gram per hour of a concentrated solution containing 48.5% (N H,) SO,. Upon cooling (NH,) SO., crystallized from the discharge concentrated solution.

The average temperature of the electrolyte was 103.7 C. measured at the electrodes and 106.8 C. measured in the central portion of the cell.

The boiling of the electrolyte occurred in the central portion of the cell; no arcing was observed in the electrode regions and no scaling took place.

EXAMPLE 7 Using the cell and electrodes as described in Example 6, the electrolyte was a dilute return acid from an electrolytic zincwinning process.

The depth of electrode immersion was 2 inches. Singlephase power at 270 volts and 42 amperes was applied to the electrodes. The current density on the immersed portion of the electrodes was 10 A/sq.in.

Return acid with a specific gravity of 1.194 was charged continuously at the electrodes from two separate feed lines at equal rates, for a total flow of 36,940 gram per hour. From the cell was discharged continuously a concentrated return acid with a specific gravity of 1.370, at a rate of 22,640 gram per hour. The composition of the feed to the cell and of the concentrated acid discharge from the cell are shown in Table 3.

TABLE 3 Specific H,SO. Zn Cl F Gravity g/l g/l mg/l mg/l Feed 1.194 109 2.41 332 360 Discharge 1.370 218 4.94 618 660 The average temperature of the electrolyte in the cell was 103.5 C. measured at the electrodes and .5 C. measured in the central portion of the cell.

The boiling of the electrolyte took place in the central portion of the cell and no arcing or scaling was observed.

EXAMPLE 8 Using the cell and electrodes as described in Example 6, a synthetic sea water with a specific gravity of 1.024 and containing approximately 23.0 g/l NaCl, 8.0 g/l Na SO 101-1 0, 1 1.0 g/l MgCl 2.2 g/l CaCl 0.9 g/l KBr and 0.2 g/l KCl was heated and concentrated; water was recovered from the vapor phase.

The depth of electrode immersion was 2 inches.

Single phase power at 270 volts and 22 amperes was applied to the electrodes. The current density on the immersed portion of the electrodes was 5.2 A/sq. in.

The seawater was charged at the electrodes from two separate feed lines at equal rates, for a total flow of 10,880 gram per hour. From the cell was discharged 3,040 gram per hour of a concentrated salt solution with a specific gravity of 1.1 10, and 7,840 gram per hour of water vapor.

The average temperature of the electrolyte in the cell was 1007' C. measured at the electrodes and 102 C. measured in the central portion of the cell. Upon cooling of the discharge from the cell, crystallization occurred in the concentrated solution.

The boiling of the electrolyte took place away from the electrodes and no arcing was observed. A small amount of scale had formed on the surface of the electrodes.

EXAMPLE 9 Using the cell and electrodes as described in Example 6, a dilute brine consisting of NaCl and KC], analyzing 3.5% Na and 4.7% K, was concentrated to the point of incipient crystallization.

The electrodes were immersed to a depth of 2 inches in the brine.

Single phase power at 200 volts and 46.5 amperes was applied to the electrodes. The current density on the immersed portion of the electrodes was 1 l A/sq.in.

The dilute brine was charged continuously at the electrodes from two separate feed lines at equal rates, for a total flow of 28,150 gram per hour. From the cell was'discharged continuously 15,960 gram per hour of a concentrat-ed brine which analyzed 6.3% Na and 8.3% K. Upon cooling, salts crystallized from the hot brine.

The average temperature of the brine in the cell was 107 C. measured at the electrodes and 109 C. measured in the central portion of the cell.

The electrolyte boiled in the central portion of the cell, away from the electrodes, no arcing was observed and, although the brine was concentrated to the point of incipient crystallization, no scaling occurred.

EXAMPLE 10 Using the cell and electrodes as described in Example 6, a caustic solution containing NaCl as impurity was concentrated in a continuous manner.

The electrodes were immersed to a depth of 3 inches in the solution.

Single phase power at volts and 58 amperes was applied to the electrodes. The current density on the immersed portion of the electrodes was 9.3 A/sq.in.

The solution, analyzing 12.7% Na and 10.5% C1, was charged at equal rates at the electrodes, for a total flow 38,588 gram per hour. From the cell was discharged 26,530 gram per hour of a concentrated solution analyzing 16.4% Na and 14.9% C1.

The average temperature of the solution in the cell was 1 1 1 C. measured at the electrodes and 114 C. measured in the central portion of the cell. No boiling of the electrolyte occurred in the electrode regions and no arcing or scaling was observed.

In the following example it is shown possible using the process and apparatus of the invention to heat or concentrate electrically non-conductive solutions which have been made conductive by the additions of a small percentage of an electrolyte. It is also shown that the gaseous phase evolved from the electrolyte can be advantageously recovered.

EXAMPLE 11 An oblong cell 12 inches long, 2 inches wide and inches high was used with a bottom canted in a way that the center of the cell was raised 1 inch above the ends of the cell. An overflow discharge port was located 2 inches from the bottom at the center of the cell.

In the cell two graphite electrodes with a cross-section of one-half inch square were positioned at opposite ends and maintaining one-quarter inch clearance between the electrodes and the walls of the cell.

The electrodes were immersed to a depth of 2% in a solution consisting of methanol, water and a small quantity of NaCl. Single phase power at 80 volts and 9 amperes was applied to the electrodes. The current density on the immersed portion of the electrodes was 1.6 A/sq.in. The temperature of the cell contents during the test was constant at 85 C. A solution composed of 33.3% methanol, 64.7% H 0 and 2.0% NaCl was continuously charged to the center of the cell at a rate of 3,635 gram per hour. A vacuum of 2.5 inches mercury was used for withdrawal of the vapors from the cell, which were condensed in a condenser. The condensed vapors flowed from the condenser at a rate of 1,005 gram per hour and had a composition of 63.9% methanol and 36.1% H O. A product with a composition of 22.0 methanol, 75.2% H 0 and 2.8% NaCl was removed continuously from the cell by a peristaltic pump, regulated to give a constant level of the liquid in the cell, at an average rate of 2,360 gram per hour. No arcing was observed and no scaling took place.

It will be understood, of course, that modifications can be made in the embodiments of the invention described and illustrated therein without departing from the scope and purview of the appended claims.

What we claim as our invention:

1. In a continuous process for electrothennally concentrating an aqueous electrolyte by boiling in a cell having at least two electrodes with a central portion located about equidistant from said electrodes, said central portion having a discharge opening, and said cell having an alternating current passing through said electrolyte between said electrodes. the improvement comprising preventing arcing and formation of vapor at said electrodes by cooling the region of said electrodes by feeding fresh electrolyte having a temperature at least 2 C. electrodes, the temperature of the electrolyte in the region of said electrolytes, increasing the temperature of said electrolyte as it flows toward said central portion and boiling said electrolyte in said central portion by selecting a cross-sec tional area for the electrolyte in said portion reduced from the cross-sectional area for the electrolyte in the electrode region such that the resulting increased current density causes boiling in said central portion, and removing the concentrated electrolyte through the discharge opening in said central portion.

2. The improvement of claim 1 wherein vapor producedby boiling is withdrawn from the cell and is recovered.

3. The improvement of claim 2 wherein the aqueous electrolyte is seawater.

4. The improvement of claim 1 wherein the aqueous electrolyte comprises methanol, methanol vapor is formed in the cell and said vapor is withdrawn from the cell for recovery of methanol.

5. The improvement of claim 1 wherein the aqueous electrolyte is wet-process phosphoric acid and concentrated phosphoric acid is continuously withdrawn through the discharge opening in the central portion.

6. The improvement of claim 1 wherein the cross-sectional area of the electrolyte in the central portion is reduced from the cross-sectional area of the electrolyte in the electrode region by a decreased depth of the electrolyte in the central portion.

UNITED STATES PATENT OFFICE v QERTIFICATE OF CURRECTION Patent No. 3,664,929 Dated May 23, 1972 n r( Arnold G. White. Thomas E. Smiiin Lvall Cl Work and Willard W. Miller It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 2, line 21, "arching" should read -arcing-; column 3, in lines 32 and 33, "3" should read in claim 1, line 14, "electrodes" should read.--]oelowand inline l5, "electrolytes" should read electrodes-.

, Signed and sealed this 12th day of December 1972.

' (SEAL) Attest:

EDWARD M.FLET(?HER,JR. I 7 ROBERT GO'ITSCHALK Attesting Officer Commissionerof Patents FORM PO-105O (10-69) USCOMM-DC 60376-P69 U.5. GOVERNMENT PRINTING OFFICE: I969 036633l UNHED STATES PATENT @FFECE QEMEFMATE @F @UHQN Patent No. 3,664,929 Dated May 23, 1972 Invent fl Arnold G. White, Thomas E. Smith, Lvall Cl Work and Willard W. Miller It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 2 line 21, "arching" should read arcing-; column 3, in lines 32 and 33, "3" should read in claim 1, line 14, "electrodes" should read -belowand in line 15, "electrolytes" should read -electrodes.

4 Signed and sealed this 12th day of December 1972.

(SEAL) Attest:

EDWARD MQFLETQHERMR. ROBERT GOTTSCHALK Attesting Officer Commissioner of Patents FORM PO-105O (10-69) I uscoMM-Dc scans-ps9 U.5. GOVERNMENT PRINTING OFFICE: 1969 0366-334 

2. The improvement of claim 1 wherein vapor produced by boiling is withdrawn from the cell and is recovered.
 3. The improvement of claim 2 wherein the aqueous electrolyte is seawater.
 4. The improvement of claim 1 wherein the aqueous electrolyte comprises methanol, methanol vapor is formed in the cell and said vapor is withdrawn from the cell for recovery of methanol.
 5. The improvement of claim 1 wherein the aqueous electrolyte is wet-process phosphoric acid and concentrated phosphoric acid is continuously withdrawn through the discharge opening in the central portion.
 6. The improvement of claim 1 wherein the cross-sectional area of the electrolyte in the central portion is reduced from the cross-sectional area of the electrolyte in the electrode region by a decreased depth of the electrolyte in the central portion. 